INTEGRAL, XMM-Newton and ESO/NTT identification of AX J1749.1-2733: an obscured and probably distant Be/X-ray binary

AX J1749.1-2733 is an unclassified transient X-ray source discovered during surveys by ASCA in 1993-1999. A multi-wavelength study in NIR, optical, X-rays and hard X-rays is undertaken in order to determine its nature. AX J1749.1-2733 is a new high-mass X-ray binary pulsar with an orbital period of 185.5+/-1.1 d (or 185.5/f with f=2,3 or 4) and a spin period of ~66 s, parameters typical of a Be/X-ray binary. The outbursts last ~12 d. A spin-down of 0.08+/-0.02 s/yr is also observed, very likely due to the propeller effect. The most accurate X-ray position is R.A. (2000) =17h49m06.8s and Dec. = -27deg32'32".5 (unc. 2"). The high-energy broad-band spectrum is well-fitted with an absorbed powerlaw and a high-energy cutoff with values NH=(20+/-1)e22 cm-2, Gamma=1.0+/-0.1, and Ecut=21+/-3 keV. The only optical/NIR candidate counterpart within the X-ray error circle has magnitudes of R=21.9+/-0.1, I=20.92+/-0.09, J=17.42+/-0.03, H=16.71+/-0.02, and Ks=15.75+/-0.07, which points towards a Be star located far away (> 8.5 kpc) and highly absorbed (NH~1.7e22 cm-2). The average 22-50 keV luminosity is (0.4-0.9)e36 erg/s during the long outbursts and 3e36 erg/s during the bright flare that occurred on MJD 52891 for an assumed distance of 8.5 kpc.